1. Field of the Invention
The present invention relates to a high-pressure discharge lamp, and more particularly to a sealed structure for a high-pressure discharge lamp, and to a method for manufacturing the same.
2. Description of the Related Art
Liquid-crystal projector devices and the like have recently been adopted as means for magnifying, projecting, and displaying characters, patterns, and other images. High-brightness, high-pressure discharge lamps are commonly used on a wide scale as light sources because such image projection devices require a certain light output. Such lamps are commonly combined with reflecting mirrors. A need has recently arisen for the distance (arc length) between the electrodes of high-pressure discharge lamps to be reduced in order to increase the light-gathering efficiency of the reflecting mirrors.
Such a reduction in the interelectrode distance, however, is tied to a drop in lamp voltage, resulting in increased lamp current if the goal is to retain the same lamp power. Increased lamp current leads to increased electrode loss, causes the electrode material to vaporize more vigorously, and promotes premature electrode deterioration, that is, reduces lamp life.
This is the reason that attempts are usually made to increase mercury vapor pressure or the like during lamp operation and to prevent lamp voltage from decreasing (lamp current from increasing) when the interelectrode distance is reduced. An example is the high-pressure mercury lamp disclosed in Japanese Laid-Open Patent Application 2-148561.
The interelectrode distance of the disclosed lamp is 1.2 mm, and the operating pressure reaches about 200 atm when the lamp is lighted at a rated power of 50 W. According to the disclosure, a high lamp voltage (76 V) can be obtained at such a high pressure.
The operating pressure must be sufficiently high in order to obtain an adequate lamp voltage when the interelectrode distance is reduced in such a manner. The lamp must therefore have a sealed structure of sufficiently high pressure resistance capable of withstanding operation at such superhigh pressures.
FIG. 39(a) is a diagram showing part of the overall structure of the sealing component of the high-pressure lamp disclosed in Japanese Laid-Open Patent Application 2-148561.
In the drawing, 100 is a roughly spherical light-emitting component made of quartz glass, 101 is a lateral tubular component extending from the light-emitting component 100, and 102 is a tungsten electrode for feeding electric current to the light-emitting component 100. 103 is a molybdenum foil; 104, a molybdenum lead wire for introducing outside current. These components constitute an electrode assembly 105, obtained by connecting the electrode 102 to one end of the molybdenum foil 103 (one end of the electrode extends into the light-emitting component 100), and the electric current feeding lead wire 104 to the other end. The electrode assembly 105 is air-tightly sealed in the lateral tubular component 101 by a sealed foil structure in which the elastic deformation of the molybdenum foil 103 is used for absorbing differences in thermal expansion in relation to the quartz glass.
An effective means capable of withstanding high operating pressures (up to 200 atm) with the aid of such a sealed foil structure is described in detail, for example, in "The 7th International Symposium on the Science & Technology of Light Sources (1995), Symposium Proceedings," p. 111.
An overview of the above publication will now be given with reference to FIG. 39(b).
FIG. 39(b) is a cross section of area AA' in FIG. 39a. Part of the electrode 102 is embedded in the lateral tubular component 101, and a non-adhesive area 107 is formed around the electrode 102, as shown by the dots in FIG. 37. According to the aforementioned article, if W is the width of the non-adhesive area 107, the resistance of the lamp to high pressure can be improved by reducing the width W of the non-adhesive area 107. Specifically, it is claimed that using a structure of smaller width W makes it possible to reduce pressure concentration in the non-adhesive area 107 and improves the resistance of the lamp to high pressure.
Meanwhile, a sealed rod structure such as that disclosed in U.S. Pat. No. 4,282,395 is a known example of another sealed structure resistant to high pressures. In this structure, as shown in FIG. 40, electrodes 102 are air-tightly sealed in lateral tubular components 101 through the intermediary of glass (for example, superhard glass 200 with a coefficient of expansion of 32.times.10.sup.-7 /.degree. C.) whose coefficient of thermal expansion lies between that of quartz glass (coefficient of expansion: 5.5.times.10.sup.-7 /.degree. C.) and that of tungsten (coefficient of expansion: 46.times.10.sup.-7 /.degree. C.), in areas B of the lateral tubular components 101 at a distance from the light-emitting component 100. The force with which the electrodes 102 and the quartz glass are bonded via the intermediate glass 200 is much greater than the force with which the molybdenum foil and the quartz glass are bonded by the elastic deformation of the molybdenum foil, providing a structure that is superior to a sealed foil structure in terms of resistance to high pressure.
According to the above-described teaching, the maximum withstand pressure of a lamp with a sealed foil structure is limited in terms of the diameter of the electrodes 102 because the diameter of the electrodes 102 is the minimum value of the width W shown in FIG. 39(b). This is the reason that using a sealed foil structure makes it difficult, for example, to construct the lamp of high operating pressure (up to 200 atm) described in Japanese Laid-Open Patent Application 2-148561 as a high-output lamp requiring a large lamp current and a sufficiently thick electrode to accommodate this current. For this reason, all the examples of lamps disclosed in Japanese Laid-Open Patent Application 2-148561 are limited to low-output lamps of 50 W or lower.
The sealed rod structure depicted in FIG. 40 can endow a lamp with higher resistance to high pressure than a sealed foil structure, making it possible to provide a lamp whose output and operating pressure are higher than those of a lamp with a sealed foil structure. The conventional sealed rod structure depicted in FIG. 40 is unsuitable, however, for lamps in which the light-emitting substance consists of mercury or another substance whose vapor pressure varies widely with the lamp temperature during operation, such as the lamps described, for example, in the aforementioned Japanese Laid-Open Patent Application 2-148561.
This is because the maximum service temperature of the intermediate glass used in the sealed rod structure is lower than the normal value of 900.degree. C. (maximum value: 1100.degree. C.) for quartz glass, as typified by the normal value of 230.degree. C. (maximum value: 490.degree. C.) for superhard glass, requiring that, normally, the electrode be air-tightly sealed near the areas B of the low-temperature portion at the maximum distance from the light-emitting component 100, which develops the highest temperature during operation, as shown in FIG. 40.
Because of this, low-temperature regions (gaps where the lateral tubular components 101 are not bonded to the electrodes 102) designated as areas A form in FIG. 40 inside the sealed light-emitting component 100 of a lamp with a sealed rod structure. Consequently, mercury is sealed as a light-emitting substance with the aid of a conventional sealed rod structure inside, for example, the light-emitting component 100, and this mercury condenses in areas A even when the goal is to obtain the mercury lamp of high operating pressure disclosed in Japanese Laid-Open Patent Application 2-148561, making it impossible to obtain the desired mercury vapor pressure or to construct a properly operating lamp. When an attempt is still made to ensure that the lamp operates properly by forming a seal near the connection between the light-emitting component 100 and the lateral tubular components 101, the intermediate glass 200 is exposed to high temperatures and is melted during operation, and the bond is broken and the lamp fractured by the difference in pressure between the high-pressure light-emitting component 100 and the outside, which has roughly atmospheric pressure.